This is another view of the long mound of rocks going up the hollow. I really don't think it was ever a wall. Having spent most of my life up in the North of England I know plenty about stone walls and their construction. It also doesn't appear to have been the foundation of a barn or other structure. But, what ever this was I think it's been damaged somewhat. Many people have lived in our old farmhouse over the last 100 years and I can imagine their kids spending many happy hours playing and scrambling around on this rock pile. I know I would have!